FIELD OF THE INVENTION
The invention relates to an auxiliary resonant commutated pole (ARCP) multi-point converter having variable potential intermediate-circuit capacitances and thus to circuits appertaining to power electronics, in particular soft-switching multipoint converter topologies for high-power converters.
Hard-switching multi-point converters having variable-potential intermediate-circuit capacitances, as have been proposed, for example, by T. Meynard et al. in International Patent Disclosure WO 93/02501, are used in the high-power field for controlling three-phase drives and, in power transmission systems, for gateways and compensation. The multi-point converter concept has been proven, in particular at high voltage levels for which the maximum reverse voltage of an individual active semiconductor device that is now available is inadequate. In contrast to the multi-point converter topologies with null or limiter diodes, as have been described, for example, by A. Nabae et al. in the reference titled "A New Neutral Point Clamped PWM Inverter", IEEE Transactions of Industry Applications, Vol-IA-17, No. 5, in the multi-point converter topology proposed by Meynard et al. the different output voltage levels on a converter path are obtained by skillful interconnection of differently charged intermediate circuit capacitances. The advantages of this topology become apparent in particular in multi-point converters having more than three points, in which the number and the required reverse voltage loading of the null or limiter diodes increase greatly in a topology in accordance with the reference by Nabae et al.
At present, GTO switches with inverse diodes are used in multi-point converters with a voltage intermediate circuit in the high power field. In this configuration, the maximum current gradients di/dt and the voltage gradients du/dt that occur have to be limited by passive limiter networks, in order to avoid destruction of the active semiconductor devices. Such networks are often highly lossy, and contribute significantly to converter complexity and converter costs. The maximum achievable switching frequency in these high-power converters is limited by the switching losses that occur in the semiconductor and by the minimum switching and recovery times of the semiconductor components. Since the switching frequency has a direct influence on the quality of the electrical input and output variables, and thus on the overall system configuration, the achievable switching frequency is a major quality criterion for a converter.
Progress in power-semiconductor development is now allowing converters to be operated with a considerably greater di/dt and du/dt, and this has resulted in the limiter networks becoming considerably smaller, or even being dispensed with. The present limit in the achievable switching frequency is thus now governed essentially only by the maximum permissible semiconductor losses.
Various soft-switching converter topologies that allow the switching losses to be significantly reduced have been proposed in order to increase the maximum switching frequency for converters in the low and medium power ranges. In particular, the "auxiliary resonant commutated pole" (ARCP) principle for two-point converters, proposed in U.S. Pat. No. 5,047,913 by R. De Doncker et. al, is highly suitable for reducing switching losses. In such an ARCP converter, a load relief capacitor is connected electrically in parallel with each main switch. Furthermore, an auxiliary circuit is provided, which contains an auxiliary switch that is electrically connected in series with a resonant inductance, and which connects a neutral point of a DC voltage intermediate-circuit capacitor to the output connection of the converter phase. All the main switches operate in the zero-voltage mode, while all the auxiliary switches operate in the zero-current mode.
In addition to the drastic reduction in switching losses, the ARCP principle also allows the maximum rate of current and voltage rise to be controlled by the choice of the resonant elements. Which, apart from the opportunity to use critical semiconductor switches or combinations of semiconductor switches (e.g. series circuit), also results in a reduction in the load on the insulation of the end turns in three-phase motors.
Approaches to extending the ARCP principle to three point converters with variable-potential capacitances have been shown by Dijkhuizen et al. at the IEEE IAS 98 Conference and by Deschamps et al. and by Yuan et al. at the Brazilian Power Electronics Conference COBEP 97. In these solutions, the converter output is connected to a resonant inductance, which is connected either to the positive or the negative DC voltage intermediate circuit rail (Deschamps) or via an additional transformer to the voltage neutral point of the DC voltage intermediate-circuit capacitor in the three point converter (Yuan). The essential disadvantage in the Deschamps configuration is that an asymmetric charge reversal operation takes place, that is to say the absolute value of the voltage across the resonant inductance at the beginning of the resonant charge reversal operation does not correspond to the absolute value of the voltage at the end of the resonant commutation. Additional lossy switching operations in the auxiliary path during the resonant commutation are necessary in order to achieve current decay in the auxiliary path. The essential disadvantage in the configuration according to Yuan is the high outlay on components. Particularly the production of the high frequency transformers in the auxiliary path, which are loaded by a high resonant current, is too complex and too expensive for broad application of this topology.